When booking Regent, our cruise line of choice, it is best to book as soon as the schedules are released. The best cruises sell out very quickly. Regent also tends to raise prices quarterly, usually only for cruises selling well. Also, as categories tend to sell out from the bottom and the top, if you want one of these it is best to purchase as soon as possible.
I have always found it possible to get a price decrease or special included in my reservation as long as cruise is not in penalty period.